After potatoes have roasted 10 minutes, remove pan from oven. With spatula, push potatoes to an end of pan to make room for fish. Place fish in pan; sprinkle with 1/2 teaspoon salt and roast with potatoes 15 minutes or until fish flakes easily when tested with a fork and potatoes are browned and tender.

Transfer fish to platter and top with horseradish sauce; sprinkle with half of dill. Toss potatoes with remaining dill; arrange on platter with fish.

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